I found on mine the issue. the explorer app is hanging when it search for files on your smartphone. for me i have around 7500 music files on my sd card and if after a reboot i used a lot the explorer app, it stop searching and use around 50% of the CPU.
Try using cpuz app to find if your cpu is used.
I resolve this bug by waiting around 20 min after using my phone (after a reboot).
Maybe you have the same issue.
I will open a thread bug on miui forum for this.